What is the meaning of the colors in the nucleotide conservation track?

The nucleotide conservation track shows scores of evolutionarily conserved nucleotides based on phylogenetic studies between species. Nucleotide conservation scores are extracted from PhastCons statistical algorithms, represented by grey color. The red color means that the indicated value of one nucleotide is higher than what the height of the stick symbolizes. The threshold is fixed at 4 (as for the UCSC). These colors are visible when viewing values in the tooltip.